They are demanding Europe-wide legislation to be in place by next year to ensure the threat doesn't worsen.

Invasive species are defined as those that are introduced accidentally or deliberately into a place where they are not normally found.

A European inventory in 2008 found more than 10,000 alien species in Europe, with 1,300 having some kind of impact. This impact was exerted either on the environment, economy or, on human health.

And numbers are on the rise. Research published this year in the journal Science found alien species in Europe have increased by 76% in the last 30 years alone.

Piero Genovesi is chair of the Invasive Species Specialist Group (ISSG), a global network of experts on invasive species. He told BBC News that the figure of 12 billion Euros represents a significant underestimate of the impact of alien species.

Huge cost"For many species we have no idea what damage they cause or their economic impact. This is just a fraction of the actual cost," he told BBC News.

And he added that this estimate does not include any assessment of the economic value of lost biodiversity caused by non-native species.

Scientists gathered at the conference are calling for urgent action by the European Union to implement laws similar to those that already exist in countries like New Zealand and Australia.

"We're asking the EU to rapidly develop and approve a policy on invasive species, fulfilling the formal commitment agreed by the council of European ministers in June 2009," Mr Genovesi told BBC News.

"This is urgent, we would like this to be in place by next year."

... The Ruddy duck was introduced to Europe as an ornamental species. It is one of the worst culprits because of its aggressive courting behaviour and willingness to interbreed with endangered, native duck species.

Feral pigs, ship rats, s, Cane Toads, and European Rabbits having nothing on the most invasive species on Earth:

Science NewsWeek of Oct. 13, 2007; Vol. 172, No. 15

Invasive, Indeed

One species-Homo sapiens-consumes nearly a quarter of Earth’s natural productivity

Sid Perkins

Some people live lightly on the land: Bedouin clans roam the deserts of the Middle East and North Africa; small groups of indigenous people follow reindeer herds across frigid Arctic terrain; and tribes of hunter-gatherers forage the plains of southern Africa and the forests of Amazonia and Papua New Guinea.

Then there’s the other 6.6 billion of us.

When we farm, clear forests, and build cities, dams, and roads, we dramatically alter the landscape. In some places, we increase the land’s productivity-measured as the amount of plant life at the base of the food chain-by adding immense amounts of water and fertilizer. New research indicates that on the whole, however, human presence significantly decreases Earth’s biological productivity. For instance, many of today’s cities occupy large patches of what had been some of the world’s most fertile land.

Of the biological productivity that remains, people are gathering an ever-increasing share, sometimes by boosting their quality of life, but often merely by dint of their burgeoning numbers. In some regions, each spanning millions of square kilometers, human activity consumes almost two-thirds of the biological productivity that would otherwise be available.

“We were surprised how intensively these regions were being affected” by human presence, says K. Heinz Erb, an ecologist at Klagenfurt University in Vienna. “Only one-third of the natural productivity isleft for all the other species.” ...

While wilderness areas remain relatively unaffected by people, other parts of the world are packed cheek by jowl with cities, farms, and other human imprints.

Southern Asia, a 6.7-million-square-kilometer region that includes India, is one of the most densely populated and heavily irrigated regions on the planet, says Erb. There, human activity co-opts about 63 percent of the area’s natural productivity each year, he and his colleagues estimate. In eastern and southeastern Europe, people appropriate about 52 percent of the land’s productivity.

At the other extreme, in Australia, central Asia, and Latin America, the percentage of productivity that ends up in human hands ranges between 11 and 16 percent. Increasing the use of fertilizers and irrigation could boost those percentages and help meet the needs of a growing world population. However, long-term irrigation sometimes renders the soil too salty for crops, and fertilizer, if used unsparingly, runs off into rivers and streams and ends up in the ocean, where it overfertilizes algae and thus creates huge zones devoid of other life. “There’s no free biomass,” Erb cautions.

In the stampede to replace fossil fuels, some scientists have proposed the large-scale cultivation of crops that can be transformed into supposedly eco-friendly biofuels. That, too, might be ecologically unwise.

“If the whole world begins to look like Iowa cornfields, we’ll have to take an even larger share of global biological production into human hands, and that leaves a lot less for other things,” says Foley. “And those other things won’t be just pretty butterflies and tigers and charismatic animals, they’ll be things that matter to us, like the things that clean our water, preserve our soils, clean our atmosphere, and pollinate our crops.” ...